Based on my experience, balun power ratings are for MATCHED conditions.
It is rare that hams use a balun in a matched condition. Thus a 1:1
balun should see 50 ohms on the input and 50 ohms on the output, while a
4:1 balun should see 200 ohms on the output and 50 ohms on the input.
In the case of a resonant folded dipole, a 4:1 balun is typically
operating in a nearly matched condition. All others combinations are
unknown and random.
I run about 500 watts on all bands. My baluns are rated at 5KW! It
takes 3 or 4 big hunkin' pieces of ferrite to attain this power level.
My 6 meter balun is a 1/2 wavelength electrically of RG-213. No ferrite!
Buy or build a balun of your choice. Using an IR temperature gun,
measure the ambient temperature of the core. Run about 1/2 rated power
carrier for 30 to 60 seconds. Measure the temperature again. If it is
warm to hot, this is RF producing heat. And likely continuing will
produce core failure. This is not a good balun for your application.
One of my baluns work between the output of my KAT500 and the balanced
feed line connected to the center of a 256 ft wire. That antenna works
160M - 6M with zero issues. Now, I do run a hybrid balun being a 4:1
Guanella balun as a transformer, and it is fed with a 1:1 balun for
common mode rejection.
Most single core, i.e. 2 or 3 cores stacked with 2 to 4 windings are not
at all a proper balun design A Guanella balun will have 2 cores with 2
windings and then another 2 separate cores with another 2 windings.
These are then wired to produce a 4:1 balun with good common mode
rejection. Most "factory" 4:1 baluns are poorly designed and built junk.
See https://www.dj0ip.de/balun-stuff/ for further references.
73
Bob, K4TAX
On 6/1/2020 8:45 AM, Alan - G4GNX wrote:
I have a similar issue with ferrite 'balun' heating, although mostly
tested on 40 metres. I'm using an OSCFD (Windom) which has a main 4:1
balun at the feed point, but also has a sleeve 'balun' consisting of 8
ferrites wrapped in a plastic sleeve, about 3 feet from the feed
point. If I run the K3S at 100W, via the KPA500 in standby, then
through the KAT500 tuner, I see no problems.
On increasing the power to 200W by using the KPA500 in Operate and
about 10W drive from the K3S, during a SSB 'over' the temperature of
the KPA500 rises and after a few minutes, the SWR readings on the
KPA500 and KAT500 start to rise. One of the Elecraft guys actually
very kindly went through my fault log and came to the conclusion that
the fault is with the antenna/feeder and I agree with him. What is
strange is that if I leave a solid carrier running at 200W, I can't
easily reproduce the issue and I wonder if its something being
affected by peak excursions with SSB?
When things do start to go wrong, I can usually perform a manual tune
with the KAT500 and the fault will then not (mostly) be apparent.
Anyway, I'm in the process of replacing the entire feeder with
Westflex 103 and much bigger ferrites. I will also replace the main
balun which is a 400W device for a 1KW device, as I think that running
the UK max of 400W may be pushing the spec of a 400W balun. Also, the
balun itself or connections to it may be faulty.
The point now is that I have discovered some damage to the outer
covering of the sleeve 'balun' which may be the result of ferrites
overheating or may be the cause of movement of the ferrites thus
causing instability. Either way, heftier devices seem to be the order
of the day.
Although the KPA500 temperature rise is somewhat expected, I feel it's
a bit higher over a short period than I would like and may be due to
reflected power.
73,
Alan. G4GNX
------ Original Message ------
From: "David Olean" <k1...@metrocast.net>
To: "Peter Dougherty" <li...@w2irt.net>
Cc: elecraft@mailman.qth.net
Sent: 01/06/2020 13:25:18
Subject: Re: [Elecraft] NOT the feedline (was KPA-1500 faulting on 6m;
SWR issues)
Hello Peter
I suspect RFI as well. I do not have a KPA1500, so my advice is not
worth much, but the fact that the fault only occurs when a signal is
being radiated sends me a red flag. I had a similar problem here
with my SPE amplifier power output. I was measuring it on an LP-100A
wattmeter. It turns out that 1.8 MHz energy from the SPE amp
radiated from my vertical antenna and was getting into a new wire
doublet antenna and coming back down the ladder line into the shack.
The ladder line was about 7 inches from the power coupler for the
LP-100A. RF caused the readings to go wacky and it looked like my
amplifier was going postal! The problem was high rf fields next to
the power meter coupler.
Ferrite balun heating can be seen with a VSWR meter if you set the
power at some intermediate level, key the amp up with a steady
carrier, and watch for VSWR creeping up. Loose pins in coax can also
be a problem. They can heat up and disconnect or start arcing. Those
problems do not typically show VSWR creep. They happen all at once
usually.
The KPA1500 works fine into a dummy load. (No RFI) That is a big
clue. Good luck and I hope you find the problem. Maybe an RF
sniffer would detect the ingress point?
73
Dave K1WHS
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